1. Field of the Invention
The present invention refers to an apparatus for fixing a workpiece to be machined by a machine tool to the worktable of said machine tool in an exactly determined positional relationship with reference to a reference point located on said worktable of said machine tool. The apparatus comprises a workpiece receiving means adapted to removably receive said workpiece to be machined, and first positioning means comprising a first group of positioning apertures provided in said workpiece receiving means and a first group of positioning studs provided on said worktable of said machining tool, said first group of positioning apertures and said first group of positioning studs co-operating to determine the position of the workpiece receiving means in X- and Y-direction.
It is often required to fix a workpiece to be machined by a machine tool in an exactly known, well defined relationship in the working area of a machine tool, particularly with reference to the worktable of a machine tool. Particularly, an exact and precisely known positioning of a workpiece to be machined must be achieved with reference to two perpendicularly running directions, i.e. the X-direction and the Y-direction, as well with reference to a rotation around the Z-axis, i.e. the axis running perpendicularly through the section of said X-axis and said Y-axis. Additionally, sometimes it is required that the workpiece be exactly positioned with reference to said aforementioned Z-axis, i.e. with reference to its height position above said X-Y-plane and with reference to its parallelity to the X-Y-plane.
2. Prior Art
It is well known in the prior art to provide workpiece positioning means adapted to position comparatively small workpieces. These positioning means comprise a workpiece receiving means adapted to receive a workpiece to be machined, as well as a base member mounted on the worktable of the machining apparatus and cooperating with said workpiece receiving means. The positioning is effected by means of protruding positioning studs provided on said base member and correspondingly arranged positioning apertures provided on said workpiece receiving means; as soon as the workpiece receiving means is coupled with the base member, the positioning studs engage the positioning apertures to center the workpiece connected to the workpiece receiving means.
Taking the positioning along the extension of the Z-axis not in account, it is basically sufficient to provide two pairs of cooperating studs and apertures; the first pair can provide a positioning in X- and Y-direction and the second pair can provide, in cooperation with the first pair, a well defined angular orientation around the Z-axis.
An essential prerequisite with such known apparatusses for the positioning of workpieces is that the positioning studs, on the one hand, and the positioning studs, on the other hand, are very precisely located as far as their relative position vis-a-vis each other is concerned. With other words, the distance between e.g. two positioning studs must be exactly equal to the distance between two positioning apertures cooperating with said positioning studs in order to achieve a flawless cooperation between positioning studs and positioning apertures and thereby the desired positioning of the workpiece receiving means with reference to the base member (the position thereof being well known). The result is that these parts must be manufactured with an extremely high degree of precision, particularly if an extremely high degree of positioning accuracy is required as it is the case in the field of electro erosive machining of workpieces. Thus, the manufacture of such a high precision positioning apparatus, even if it is suited only for small workpieces, is very expensive and time-consuming. In addition, such a known arrangement is often geometrically overdefined, particularly if as in most cases conically shaped positioning studs are used because in this case both stud-aperture-pairs determine the X-Y-position independently from each other.
These problems and disadvantages are particularly important in the case if relatively big workpieces have to be exactly positioned. Basically, in the interest of a stable positioning and fixing of the workpiece, the distance between the positioning studs and positioning apertures, respectively, is choosen as great as possible. However, even in the region of distances between 20 and 40 cm between the stud-aperture-pairs, deviations may be observed which are caused by thermic contraction or expansion due to thermal differences between the workpiece receiving means and the base member. Thus, a flawless engagement of one or the other positioning stud in its related positioning aperture is impaired. The result is that the high precision particularly required in the manufacture of precision tools is no longer warranted.
Another problem may be observed when different workpiece receiving means are to be connected to one and the same base member. The workpiece receiving means probably have been manufactured in different manufacturing batches and, therefore, do not have exactly the same dimensions. Even if such deviations in dimension might be small, they nevertheless can severely impair the precision of the positioning of the workpiece receiving means with reference to the exactly positioned base member, an impairment which can not be tolerated in precision tool manufacturing.
The British Patent No. 2 033 263 discloses an apparatus for clamping a workpiece on the worktable of a machine tool in an exactly defined position. Said apparatus comprises a workpiece receiving means to which the workpiece to be machined is releasably fixed and which is provided with positioning recesses cooperating with positioning studs correspondingly arranged on the worktable of the machining apparatus and projecting over the surface thereof. This apparatus comprises, in particular, a workpiece receiving pallet having four V-grooves arranged in the region of the four corners of said pallet; these V-grooves essentially correspond to the positioning apertures mentioned hereinbefore. The V-grooves cooperate with correspondingly arranged V-blocks in order to position the pallet in X- and Y-direction. Thus, a group of four aperture-stud-pairs is disclosed being arranged in relatively great distance from each other and which perform the entire positioning function. It is evident for every person skilled in the art that the positioning accuracy of the pallet on the worktable of the machine tool solely depends on the manufacturing precision and on the actual position of the V-grooves and the V-blocks. The design is geometrically overdefined, and even the least deviation in the position of one of the V-grooves in the pallet will impair the exact position of the pallet with reference to the worktable of the machine tool. Such a deviation may easily occur if pallets are used which origin from different manufacturing batches or which have different temperature.
The German Patent No. 31 15 586 discloses an apparatus for exchanging and positioning pallets in a machine tool; said apparatus comprises a delivering track on which the pallets loaded with a workpiece are fed to a pallet receiving and workpiece machining station. After the pallet having reached this station, the pallet is lowered until it rests on the top surface of Z-axis centering pins which, however, do not have any centering or positioning function in X- or Y-direction. On the other hand, during the lowering of the pallet, the top portions of the conical centering pins engage correspondingly conically designed centering bushes. These bushes are resilient in axial direction, but rigid in radial direction. It is said in the aforementioned German Patent that thereby "an exact positioning of the pallet on the centering pin in radial direction" should be achieved.
However, as should be clear to every person skilled in the art, it is not possible to obtain an exact positioning of the pallet in X- and Y-direction nor with reference to the angular orientation around the Z-axis with this design, particularly if the distance between the centering pins and the distance between the centering bushes is not exactly equal. If the aforementioned distances are not exactly equal, the position of the pallet is not well defined because, depending on the initial position of the pallet prior to lowering it, either the one pin-bush-pair or the other pin-bush-pair exactly engages and determines the position of the pallet. The result is that the pallet can take two different positions; a condition which should be avoided by the present invention.